This invention relates generally to temperature measuring systems and more particularly to optical switches used in such systems.
A gas turbine engine includes a compressor that provides pressurized air to a combustion section where the pressurized air is mixed with fuel and burned for generating hot combustion gases. These gases flow downstream to a multi-stage turbine. Each turbine stage includes a plurality of circumferentially spaced apart blades or buckets extending radially outwardly from a wheel that is fastened to a shaft for rotation about the centerline axis of the engine. The hot gases expand against the turbine buckets causing the wheel to rotate. This in turn rotates the shaft that is connected to the compressor and may be also connected to load equipment such as an electric generator or a gearbox. Thus, the turbine extracts energy from the hot gases to drive the compressor and provide useful work such as generating electricity or propelling an aircraft in flight.
It is well known that the efficiency of gas turbine engines can be increased by raising the turbine operating temperature. As operating temperatures are increased, the thermal limits of certain engine components, such as the turbine buckets, may be exceeded, resulting in reduced service life or even material failure. In addition, the increased thermal expansion and contraction of these components adversely affects clearances and their interfitting relationship with other components. Thus, it is desirable to monitor the temperature of turbine buckets during engine operation to assure that they do not exceed their maximum rated temperature for an appreciable period of time.
A common approach to monitoring turbine bucket temperature is to measure the temperature of the gas leaving the turbine and to use this as an indication of the bucket temperature. The turbine exit temperature can be measured by locating one or more temperature sensors, such as thermocouples, in the exhaust stream. Because the bucket temperature is measured indirectly, it is relatively inaccurate. Thus, it does not permit optimum bucket temperatures to be utilized because a wide safety margin must be maintained.
The drawbacks of indirect bucket temperature measurement are well known, and approaches for measuring, bucket temperatures directly have been proposed. One direct measurement approach uses a radiation pyrometer located outside of the engine casing and having a field of view focused on the turbine buckets through a sight glass formed in the casing wall. Radiation emitted by the heated turbine buckets thus impinges on the pyrometer that then generates an electrical signal representative of the bucket temperature. However, during engine operation the sight glass is exposed to high temperature exhaust gases that tend to cloud the sight glass and adversely affect the pyrometer reading. Furthermore, the optical emissivity of the bucket surfaces is usually unknown, which also introduces error into the temperature measurement.
Accordingly, it would be desirable to have an approach to monitoring turbine bucket temperature that remotely monitored bucket temperature through the available sight glass, while avoiding the problems of limited optical access, impaired sight glasses, and unknown surface characteristics.
The above-mentioned need is met by the present invention, which provides a system for measuring bucket temperature that includes a plurality of optical detectors, such as a pyrometer and a spectrometer, and an optical switch for selectively directing radiation from turbine engine sight glass to any one of the optical detectors along a common line of sight.
In one preferred embodiment, the optical switch includes first and second blocks, with the optical detectors being disposed in the second block. A rotor is mounted between the first and second blocks for rotation about a rotational axis, and a fiber optic cable having a first end located on the rotational axis and a second end offset from the rotational axis is provided. Rotation of the rotor selectively positions the second end of the fiber optic cable adjacent to any one of the optical detectors.